1. Field of the Invention
The present invention relates to a process for making holograms. The present invention also concerns holography devices.
2. Description of Related Art
Holograms are made by illuminating a reference beam and an object beam onto a photosensitive material to produce interference fringes therein.
Multiple holograms are made by exposing a photosensitive film with a large area S.sub.o to constructing beams to form a hologram with a large area and then splitting the film into hologram elements each with a small area S. Namely, referring to FIG. 1, a reference beam 1 is illuminated from the right onto the photosensitive film 100 with a large area S.sub.o and an object beam 2 is illuminated onto the photosensitive film from left to produce interference fringes 111 to 116 (112 to 115 not shown) in the film and thereby obtain a large hologram 100, which is then divided into a number of holograms 101 to 106 with a small area S.
This process of splitting a large hologram into a number of small holograms involves the following problems.
First, the constructing beams are non-uniform in the plane of the photosensitive film, so that the diffraction efficiency of the hologram is not uniform in the plane of the large hologram.
Second, the power of a laser per unit area of the photosensitive film is reduced due to the large area of the film, but the energy necessary for exposing the unit area of the film is constant, not decreased, and, therefore, the time period for exposing the film must be elongated, thereby the path length of the constructing laser beam may be varied, making interference fringes fuzzy.
Third, if the object beam is a diverging light, the split holograms have interference fringes having different directions from each other, so that when the holograms are used in a holography device with a preliminarily fixed constant reconstructing light, the reconstructed images are different in position depending on the holograms. An example of such a holography device is a head-up display used for an automobile, described below.
Fourth, prisms and lenses used for the exposure of the photosensitive film become larger, thus increasing the cost.
An object of the present invention is to provide a process for making a number of uniform holograms in one exposure step.
The use of a heads-up display for displaying speed, etc., in front of a windshield of an automobile has attracted attention. A holography device utilizing the wavelength-selective diffraction and reflection characteristics of a hologram element has been proposed for the head-up display.
Referring to FIG. 2, in the heads-up display 10, a beam 20 involving an image for display is emitted from a display unit 30, and diffracted and reflected by a holography device 40. The diffracted and reflected beam 21 is then reflected by a deposited layer 311 on a windshield 31. A driver 32 senses the reflected beam 211 from the deposited layer 311 and can view a virtual image 212 in front of the windshield 31. Referring to FIG. 3, in the heads-up display device, the display unit 30 includes an incandescent lamp 301, a liquid crystal panel 302 disposed in front of the incandescent lamp 301, a reflecting mirror 303 and a holography device 40. The liquid crystal panel 302 can display images 212 of speed, master warnings, turning indications, maps and so fourth. The display unit 30 is generally disposed near an instrument panel 33.
Referring to FIG. 4, in the holography device 40, a beam 20 involving an image for display is incident through a cover plate 42 on the hologram element 41, by which the beam 20 is diffracted and reflected. The reflected beam is emitted from the cover plate 42 as the reconstructing beam 21. Interference fringes with predetermined diffraction and reflection characteristics have been previously formed in the hologram element 41. In FIG. 4, 22 denotes a beam reflected at the surface of the cover plate 42, 431 and 470 denote a sealant, and 44 denotes a bottom cover plate. In a holography device 40 of a heads-up display, for example, a hologram element 41 having the wavelength selectivity as shown in FIG. 5 and a reflecting characteristic as of a concave mirror is often used.
Referring back to FIG. 4, the surface reflecting beam 22 causes a noise image disturbing the normal image 212 to be displayed (FIG. 2). Other causes for a noise image include scattering of light transmitting through the hologram element 41, noise images recorded in the hologram element 41, and so fourth. Nevertheless, the main noise source is the surface reflecting beam 22.
It has been proposed to provide an anti-reflection film on the surface of the cover plate 42 to reduce the surface reflecting beam 22 reflecting at the surface of the cover plate 42. This anti-reflecting film may be very effective. For example, an anti-reflecting film of a single layer of MgF.sub.2, TiO.sub.2, ZrO.sub.2 or the like or of a multilayer of a combination thereof can reduce the noise rate from about 4%, which is typical in a conventional device, to about 0.3%. Nevertheless, there still remains a noise rate of about 0.3% and, moreover, the anti-reflecting film is expensive and is not resistant to abrasion.
The second object of the present invention is to provide a holography device with a reduced noise image, without using an expensive and non-durable anti-reflecting film.
Referring to FIG. 4, 431 denotes a sealant to optically seal the upper cover plate 42 and a substrate 411 of the hologram element 41, and 470 denotes a sealant to optically seal the lower cover plate 44 and the hologram element 41. The components of the holography device 40, i.e., the upper and lower cover plates 42 and 44, the substrate 411, the hologram element 41 and the sealants 431 and 470, are made of materials having almost the same refractive indexes, to prevent noise beams by reflections. The cover plates 42 and 44 and the substrate 411 are glass, the sealants 431 and 470 are an adhesive of epoxy resins, acrylic resins, UV-curable resins, or the like.
The reason for the frequent use of the above adhesive is that the sealant resins are cured by heating or UV irradiation so that the hologram element and the glass are adhered with each other, which is advantageous. Nevertheless, the resin sealants used in conventional hologram devices have the following drawbacks.
First, the viscosity is high, e.g., about 500 cP for an epoxy resin, so that air may be easily entrained and the refractive index of the sealant is thus varied to cause noise light.
Second, the refractive index is essentially fixed for each material. For example, the refractive index of an epoxy resin is almost constant, about 1.55, which makes the complete adjustment with the substrate and cover plates difficult and causes noise light by reflection.
Third, heat treatment or UV irradiation treatment is necessary and the reaction may be exothermic. For example, an epoxy resin should be heated to about 100.degree. C. and the temperature increases to about 150.degree. C. by the reaction heat. As a result, the sealant may react with a photosensitive material of the hologram element to form a layer of a reaction product having a refractive index different from those of the sealant and the hologram element. For example, when the photosensitive material comprises a photo-polymer such as polyvinyl pyrolidone, poly(N-vinyl carbazol) as a matrix polymer, a reaction product layer having a different refractive index is formed to cause white clouding, noise light, etc. A barrier layer should be provided on the surface of photosensitive layer in order to prevent the formation of the above reaction product layer.
Fourth, the resin sealant, upon being cured, may be partially peeled off from the cover plate or from the hologram element, which makes the refractive index irregular, and noise light appears.
The third object of the present invention is to provide a holography device in which the refractive indexes of the optical components can be easily matched with each other and deterioration of the photosensitive material is avoided.